The present invention relates to a process for the separate recovery of phosphate and carbonate minerals from finely-divided phosphate-carbonate-silicate ores or concentrates in which the carbonate to phosphate ratio is over 1, in which case an anionic collector agent is added to an aqueous slurry of these ores or concentrates and the slurry is possibly exposed to froth-flotation in order to separate a silicate-bearing residue from the combined phosphate-carbonate concentrate.
It is known that carbonate and phosphate minerals can be froth-flotated to produce a combined concentrate, using a fatty-acid type collector agent, the silicates remaining in the residue. The yield of carbonate and phosphate minerals in the combined concentrate can be improved, and at the same time the quantity of collector agent can be decreased by using various emulsifiers, neutral oil, or hot preparation in connection with the pre-flotation or grinding. By repeating the froth-flotation of the pre-concentrate several times, a combined concentrate with a purity degree of 90-95% has been obtained. The yields of carbonates and phosphates in this product have been high, 85-90% (carbonates) and 95-98% (phosphates).
When phosphates and carbonates have been froth-flotated selectively to produce separate concentrates, as a result of the high yield the phosphate concentrate has, however, contained carbonates and the carbonate concentrate has contained phosphates.
The separate recovery of carbonates and phosphates from a combined concentrate or the purification of a phosphate or carbonate concentrate obtained by selective froth-flotation causes difficulties and is unselective in conventional processes, which results in relatively high losses of phosphate in the carbonate concentrates. Neither is the concentration of the phosphate concentrate satisfactory, owing to the carbonates remaining in it.
U.S. Pat. No. 3,259,242 discloses a process for exposing a phosphate-carbonate-silicate ore to anionic froth-flotation at a pH value above 7 in order to recover a combined phosphate-carbonate concentrate and to treat this combined concentrate with an acid in order to remove the anionic froth-flotation reagent so that the combined concentrate can thereafter be exposed to cationic froth-flotation at a pH above 7 in order to produce a phosphate concentrate substantially devoid of carbonate. This process can be applied to the treatment of ores with a relatively low carbonate to phosphate ratio, at maximum 1. The process according to this patent is not, however, suitable for the selective froth-flotation of ores which contain substantially more carbonate than phosphate.
In the U.S. Pat. Nos. 3,462,016 and 3,462,017 it has been disclosed a method, in which a combined carbonate-phosphate concentrate, froth-flotated by fatty-acid, is treated with H.sub.3 PO.sub.4 or with NH.sub.4 H.sub.2 PO.sub.4. Herethrough the phosphates become inactive to froth-flotation, whereas the carbonates are froth-flotated in spite of the treatment. In this way the carbonate concentrate and the phosphate concentrate can be separated from the combined carbonate-phosphate concentrate.
It is furthermore known a method (Inst. Mining Met. Trans., Sect. C 75 (718), 233-9, 1966), in which it is used analogically with the former ones NaH.sub.2 PO.sub.4 or CaHPO.sub.4 as the depressor of phosphate minerals in the separation of carbonate and phosphate concentrates from their combined concentrate.
A disadvantage of the above-mentioned methods is their poor economy (H.sub.3 PO.sub.4 and its salts are relatively expensive) and their low selectivity, as the phosphates having better froth-flotating properties are depressed and the carbonates having slighter froth-flotating properties are froth-flotated. From this naturally follows a low selectivity, as a part of the phosphates is always rising to the carbonate concentrate and a part of the carbonates remain unflotated in the phosphate concentrate.
The object of the present invention is to provide a process for the separate recovery, with a good yield, of phosphate and carbonate minerals from phosphate-carbonate-silicate ores and concentrates which contain more carbonate than phosphate. The selective froth-flotation of phosphate-carbonate-silicate ores and concentrates which contain more carbonate than phosphate has proven to be highly problematic, and the object of the present invention is to overcome these difficulties.